The present invention relates to a continuous integrated process for producing pure terephthalic acid, and, more particularly, to an integrated process having improved means for recovering and/or recycling terephthalic acid precursors, residual acetic acid and other reaction by-products, such as methyl acetate.
Terephthalic acid is produced commercially in a two-stage process which begins with air (molecular oxygen) oxidation of paraxylene in the presence of a metal bromide catalyst system in acetic acid solvent. A crude, i.e., impure, terephthalic acid product is isolated from a slurry in the oxidation stage, usually as a dry crystalline powder. The crude terephthalic acid is recovered from the slurry in the form of a wet cake which is washed as necessary with acetic acid or water. The wet cake is then sent to a dryer where any adherent solvent is removed to form crude terephthalic acid. Water is produced as a significant by-product of the oxidation reaction and is removed from the reaction zone in a reaction off-gas stream. The off-gas stream also includes acetic acid and low levels of methyl acetate, which is a reaction by-product which can result from oxidation of acetic acid. A preferred means for separating and recovering the acetic acid from the off-gas stream for recycle is through azeotropic distillation of the off-gas stream condensate using an organic entrainer selected from, for example, n-butyl acetate, n-propyl acetate and isobutyl acetate. In such cases where the main feed to the azeotropic distillation process is derived from the oxidation reaction overheads, the presence of methyl acetate in the feed stream can adversely affect the amount of water which can be removed from the condensate azeotropically because methyl acetate's water azeotrope lies in the single phase region, i.e. its water azeotrope will have a low water content.
In the second stage of the process, i.e., the purification stage, crude terephthalic acid crystals are dissolved in water at elevated pressure and temperature, and the resulting solution is subjected to hydrogenation in the presence of a Group VIII Noble metal hydrogenation catalyst. The purified acid is recovered by crystallizing the acid from the hydrogen treated aqueous solution. A majority of the principal impurities, which are p-toluic acid derived from the compound 4-carboxybenzaldehyde and unidentified color bodies, along with some other organic components, such as benzoic acid and residual terephthalic acid, remain dissolved in the aqueous solution. This aqueous solution which remains is referred to as "pure plant mother liquor", i.e., PPML. More recent commercial two-stage processes, however, have sought to eliminate the need to recover the crude terephthalic acid as a separate dry product. Instead, the process has undergone degrees of integration whereby crude terephthalic acid crystals can be recovered from the slurry formed in the oxidation stage as a wet cake by depositing the slurry on a moving band of filter material. The wet cake is then washed with water or other solvent according to a predetermined series of washing steps, and then it can be re-dissolved almost immediately in water for purification without the need for a separate drying step.
The integrated process allows for improved economy where it is possible to recover and recycle the resulting pure plant mother liquor. However, under some operating conditions a residual amount of acetic acid can "slip" through the filtration/solvent exchange process, i.e., residual amounts retained within the recovered crude acid, and find its way into the aqueous mother liquor solution. The presence of acetic acid in the pure plant mother liquor can be problematic in attempting to recycle it for use elsewhere in the process. Hence, a method is needed to account for residual acetic acid and methyl acetate levels in the process as well as to provide for recycling residual terephthalic acid, acid precursors and pure plant mother liquor to achieve improved economy from an integrated process.